2.1 Vaccines
Vaccines have traditionally consisted of live attenuated pathogens, whole inactivated organisms or inactivated toxins. In many cases, these approaches have been successful at inducing immune protection based on antibody mediated responses. However, certain pathogens, e.g., HIV, HCV, TB, and malaria, require the induction of cell-mediated immunity (CMI). Non-live vaccines have generally proven ineffective in producing CMI. In addition, although live vaccines may induce CMI, some live attenuated vaccines may cause disease in immunosuppressed subjects. As a result of these problems, several new approaches to vaccine development have emerged, such as recombinant protein subunits, synthetic peptides, protein polysaccharide conjugates, and plasmid DNA. While these new approaches may offer important safety advantages, a general problem is that vaccines alone are often poorly immunogenic. Therefore, there is a continuing need for the development of potent and safe adjuvants that can be used in vaccine formulations to enhance their immunogenicity. See, e.g., Edelman, Molecular Biotech. 21: 129-148 (2002); O'Hagan et al., Biomolecular Engineering, 18: 69-85 (2001); Singh et al., Pharm. Res. 19(6): 715-28 (2000) for detailed review of the state of the art in vaccine development.
Traditionally, the immunogenicity of a vaccine formulation has been improved by injecting it in a formulation that includes an adjuvant. Immunological adjuvants were initially described by Ramon (1924, Ann. Inst. Pasteur, 38: 1) “as substances used in combination with a specific antigen that produced a more robust immune response than the antigen alone.” A wide variety of substances, both biological and synthetic, have been used as adjuvants. However, despite extensive evaluation of a large number of candidates over many years, the only adjuvants currently approved by the U.S. Food and Drug administration are aluminum-based minerals (generically called Alum). Alum has a debatable safety record (see, e.g., Malakoff, Science, 2000, 288: 1323), and comparative studies show that it is a weak adjuvant for antibody induction to protein subunits and a poor adjuvant for CMI. Moreover, Alum adjuvants can induce IgE antibody response and have been associated with allergic reactions in some subjects (see, e.g., Gupta et al., 1998, Drug Deliv. Rev. 32: 155-72; Relyveld et al., 1998, Vaccine 16: 1016-23). Many experimental adjuvants have advanced to clinical trials since the development of Alum, and some have demonstrated high potency but have proven too toxic for therapeutic use in humans. Thus, an on-going need exists for safe and potent adjuvants.
Cancer vaccines have been a subject of much attention. Recently, there appears to be an emerging consensus that cancer vaccines are less likely to be successful in the context of high tumor buden/load (see, e.g., Nature Medicine Commentary, 10(12): 1278 (2004) and Cancer Immunol. Immunother., 53(10): 844-54 (2004)). This is attributed to effective tumor-mediated immune suppression due to the secretion of IL-10, TGF-b, and PGE-2, among others.
On the other hand, recent evidence suggests that immediately after tumor resection or ablation, there is leakage of tumor cells in the peripheral blood. Therefore, the presence of tumor antigen in the context of low tumor burden, without associated immune suppression, may enable re-priming of the immune response. Thus, a need exists for an agent that promotes the long-term anti-tumor immunity, possibly through Th1 type cellular immune responses.
2.2 Regulatory T Cells (Treg Cells)
Treg cells refer to a population of specialized T cells that express CD4 and CD25. Treg cells are exceptional in that their main function appears to be suppression of function of other cells. In this regard, Treg cells are also referred to as “suppressor cells.” It has been reported that a further defining characteristic of Treg cells is their expression of the transcription factor Foxp3.
Due to the variety of their effect, Treg cells have been a subject of a great deal of interest. It has been reported that Treg cells may influence the outcome of infection, autoimmunity, transplantation, cancer and allergy. It has been suggested that the modes of suppression employed by Treg cells range from the cytokines IL-10 and TGF-β to cell-cell contact via the inhibitory molecule CTLA-4. Recently, it has been reported that dendritic cells (DC) may induce the activation and proliferation of Treg cells, although DC are recognized as powerful activators of immune response due, in part, to their potency as antigen presentation cells (APC). See Yamazaki et al., J. Exp. Med., 198: 235 (2003).
Generally, it is believed that Treg cells suppress the immunity of the host, and thus preventing an immunogen (e.g., a vaccine) from invoking effective immune response in the host. On the other hand, the absence of Treg cells can lead to an outburst of immune response, often resulting in inflammation or autoimmunity. Therefore, to maximize the immunity acquired from an immunogen, a balance needs to be achieved with regard to the level or functionality of Treg cells.
2.3 Gamma Delta (γδ) T Cells
Human T cells bearing the γδ T cell receptor represent a unique lymphocyte population with characteristic tissue distribution, being present in organized lymphoid tissue as well as skin- and gut-associated lymphoid tissue. γδ T cells are activated in a non-MHC restricted manner by small phosphorylated non-peptidic metabolites, including the prototypic ligand isopentenyl pyrophosphate (IPP). Some γδ T cell ligands are microbial intermediates from the farnseylpyrophosphate synthesis pathway, which is ubiquitous and essential for cell survival. This unique antigen specificity has been suggested to be best suited for activation of sentinel cells independently of antigens derived from individual microbes (De Libero, Immunology Today, 18: 22-26 (1997)). Recent data suggest that γδ T cells play a role in tumor surveillance, for example, of spontaneous B cell lymphomas (Street et al, J Exp Med, 199: 879-884(2004)), since these cells have been shown to recognize intermediates of the melavonate pathway, an essential pathway leading to cholesterol biosynthesis (Gober et al, J Exp Med, 197: 163-168 (2003)). These γδ T cell tumor ligands can be enhanced by treatment with amino-bisphosphonates (nitrogen containing bisphosphoante drugs include pamidronate and zolodronate and are used in myeloma treatment), suggesting that pretreatment with these drugs could sensitize tumor cells to γδ T cell-mediated killing. γδ T cells may also be able to augment anti-tumor immunity by enhancing dendritic cell maturation (Ismaili et al, Clin Immunol, 103: 296-302 (2002)).
In non-cancer settings, γδ T cells play a role in protection from viral infection, e.g., West Nile virus (Wang et al, J Immunol, 171: 2524-2531(2003)). Also, intraepithelial γδ T cells play a protective role in intestinal inflammation (Chen et al, Proc. Natl. Acad. Sci. U.S.A., 99: 14338-14343 (2002); and Inagaki-Ohara et al, J Immunol, 173: 1390-1398 (2004)). Furthermore, γδ TCR-bearing dendritic epidermal cells play a role in wound repair (Jameson et al, Science, 296: 747-749 (2002)).
2.4 Immunomodulatory Compounds
A number of studies have been conducted with the aim of providing compounds that can safely and effectively be used to treat diseases associated with abnormal production of TNF-α. See, e.g., Marriott, J. B., er al., Expert Opin. Biol. Ther. 1(4):1-8 (2001); G. W. Muller, et al., Journal of Medicinal Chemistry, 39(17): 3238-3240 (1996); and G. W. Muller, et al., Bioorganic & Medicinal Chemistry Letters, 8: 2669-2674 (1998). Some studies have focused on a group of compounds selected for their capacity to potently inhibit TNF-α production by LPS stimulated PBMC. L. G. Corral, et al., Ann. Rheum. Dis., 58 (suppl I): 1107-1113 (1999). These compounds, which are referred to as IMiDs® (Celgene Corporation) or Immunomodulatory Drugs, show not only potent inhibition of TNF-α but also marked inhibition of LPS induced monocyte IL1β and IL12 production. LPS induced IL6 is also inhibited by immunomodulatory compounds, albeit partially. These compounds are potent stimulators of LPS induced IL10. Id. Particular examples of IMiDs® include, but are not limited to, the substituted 2-(2,6-dioxopiperidin-3-yl)phthalimides and substituted 2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindoles described and claimed in U.S. Pat. Nos. 6,281,230 and 6,316,471, both to G. W. Muller, et al.